Echo ultrasound is a popular modality for imaging structures within the human body. One or more ultrasound transducers are utilized to project ultrasound energy into the body. The energy is reflected from impedance discontinuities associated with organ boundaries and other structures within the body; the resultant echos are detected by one or more ultrasound transducers (which may be the same transducers used to transmit the energy). The detected echo signals are processed, using well known techniques, to produce images of the body structures.
The peak pressure in the emitted ultrasound beam is related to the grey-level distribution in the resultant image. The cross-section of the ultrasound beam emitted by a transducer is described by the emission directivity function which, at any distance from the transducer, is defined as the variation of peak pressure as a function of lateral distance to the beam axis. The directivity function of a transducer is used to characterize its spatial resolution as well as its sensitivity to artefacts. The main lobe width of the beam is a measure of the transducer's spatial resolution and is characterized by the full-width-at-maximum (FWHM) of the directivity function. The off-axis intensity is a measure of the sensitivity of the transducer to artefacts. The width of the emission directivity function at -25 dB (denoted FW25) is a good measure of the offaxis intensity characteristics of a transducer in a medical ultrasound imaging system. It indicates the width of the image of a single scatterer. In a typical echo system, the -25 dB level for emission corresponds to about the preferred 50 dB dynamic range of the image.
The directivity function of a transducer is related to its aperture function (which is the geometric distribution of energy across the aperture of the transducer). The prior art has recognized that, in narrowband systems, the far-field directivity function corresponds to the Fourier transform of the aperture function; this relationship has been applied for beam-shaping in radar and sonar systems. This relationship does not hold true, however, in medical ultrasound systems which utilize a short pulse, and thus a broad frequency spectrum, and which usually operate in the near-field of the transducer. Therefore, in medical ultrasound applications the directivity function of a transducer must be rigorously calculated or measured for each combination of transducer geometry and aperture function. The directivity function of a transducer may, for example, be calculated on a digital computer using the approach set forth in Oberhettinger On Transient Solutions of the "Baffled Piston" Problem, J. of Res. Nat. Bur. Standards-B 65B (1961) 1-6 and in Stepanishen Transient Radiation from Pistons in an Infinite Planar Baffle, J. Acoust. Soc. Am. 49 (1971) 1629-1638. One applies a convolution of the velocity impulse response of the transducer with the electrical excitation and with the emission impulse response of the transducer.
A transducer may be apodized, that is: its off-axis intensity characteristics can be improved, by shaping the distribution of energy applied across the transducer to a desired aperture function. For a single disc, piezoelectric transducer, this has been accomplished by shaping the applied electric field through use of different electrode geometries on opposite sides of the disc as described, for example, in Martin and Breazeale A Simple Way to Eliminate Diffraction Lobes Emitted by Ultrasonic Transducers, J. Acoust. Soc. Am. 49 No. 5 (1971) 1668, 1669 or by applying different levels of electrical excitation to adjacent transducer elements in an array. However the method of Martin and Breazeale is limited to a number of simple aperture functions and the use of separate surface electrodes requires complex transducer geometries and switching circuits.